It is known that during the sulphonation or sulphation of organic aromatic compounds, the final product desired tends to be highly contaminated by compounds coming from secondary reactions, said contaminating compounds are of course to be separated from the desired reaction product and that means reduction in yield and additional costs in processing. When on the other hand said contaminating compounds are left in the reaction product so as to save the separation costs, quality of said reaction product changes for the worse as said by-products modify the characteristics and purity thereof and particularly its smell, colour and solubility.
Such a condition occurs when oleum is used as the sulphonating agent, but increases considerably if, according to the latest technique, gaseous sulphur trioxide is used as sulphonating agent, the SO.sub.3 is diluted in an inert carrier gas to reduce, at least partly, the reactivity thereof.
Particularly, when sulphonating aromatic compounds, a part of the amount of the fed SO.sub.3 tends to bind itself, instead of to a single molecule of the aromatic compound to be sulphonated, to two molecules, thus forming a complex molecule of the compounds to be sulphonated, in general called a sulphone, said by-product when kept in the sulphonation mixture may itself undergo a further sulphonation process, giving sulphonated sulphones.
So, when sulphonating toluene, the desired final product consists of the toluene sulphonic acid (to be afterwards neutralized when required), while the undesired by-products are sulphones or sulphonated sulphones and other compounds.
Undesired compounds of the same nature are formed also when sulphonating benzene, xylene and similar aromatic hydrocarbon compounds.
Since, as said above, said by-products are undesirable, various attempts were made to limit their production during the sulphonation reaction. It is known for instance to this end to use small amounts of acetic acid which does not eliminate completely the production of those undesired compounds which at the end remain, though in small amounts, in the final product and must be separated therefrom. Further, there is a continuous loss of said acetic acid inhibiting agent, and said loss, though small in percent with respect to the treated product, is, at the end, considerable in absolute value when the process is performed in an industrial plant.
Up to date no method, applicable on an industrial scale to permit the production of sulphonated aromatic compounds with only a negligible formation of said undesired compounds, appears to be known.